There has been much debate about the effectiveness of different sized reserves for nature conservation. However, in human-dominated landscapes such as cities, conservation reserve systems are, by necessity, often determined without using conservation planning principles. This can result in reserve systems that are small, fragmented and disconnected. In this study, we conducted a floristic survey of 68 urban grassland conservation reserves to assess how reserves of different areas contribute to species conservation. Species accumulation curves, species-area relationships, and proportion of native cover were explored. We found that 87% of all native plant species were found in small reserves< 10 ha in size, more small reserves contained a greater number of species than few large reserves of a comparable area, and cover of native species in small reserves was no different than cover in large reserves. However, large reserves harboured more uncommon species than smaller reserves. This research has several important implications for conservation planning and design, highlighting that both small and large reserves can help to conserve native plant species in urban areas. However, a preference for large reserves over small ones is embedded in much conservation planning and management. There is a danger that a normative belief that large reserves are ‘good’ and small reserves are ‘bad’ has become entrenched in conservation thinking. While the theory and evidence showing the conservation benefits of large reserves over small reserves for some organisms is clear, in some cases, small reserves can make a substantial and genuine contribution to conservation outcomes.